1. Field of the Invention
The present invention relates to a current limiting breaking device applicable to high voltage, heavy current electric circuits.
2. Description of the Related Art
When a short circuit or a grounding accident occurs on a circuit, several tens of kA accident electric current flow, causing severe damage to the power system and electric power apparatus. Technology which instantaneously detects and suppresses this kind of accident electric current is termed current limiting technology and up to now, devices based on various principles have been developed. FIG. 1 and FIG. 2 show the representative, conventional technology of the construction and application circuits of these current limiting breaking devices. (Japanese Examined Patent Publication (Tokkoushou) No. 50-4876). In the application circuits of FIG. 1, E is 3-phase electric power source, R and X are resistance and inductance provided on the short circuit, B is a breaking device and RG is a current limiting component. In this conventional technology, a PTC (Positive Temperature Coefficient) resistor consisting of niobium carbide is used as current limiting component RG, and the accident electric current is current limited utilizing the special characteristics of the FTC. As shown in FIG. 2, niobium carbide has the special PTC characteristic of its inherent resistance which changes considerably in response to rises in temperature. Therefore, with normal electrical current values, as the inherent resistance of the current limiting component RG is small, the calorific value is also small and the temperature of the current limiting component RG does rot rise, maintaining a low resistance state and does not affect the circuit. However, the heat generated in the current limiting component RG increases rapidly and the component temperature rises, when an accident occurs and an excessive electric current flows. As a result, the inherent resistance of the current limiting component RG increases and acts by reducing the circuit current. By means of this type of current limiting operation, as the accident electric current is suppressed to a large extent, it becomes possible to miniaturize the breaker device and reduce damage to the power system caused by accident electric current.
As in the above, by constructing conventional current limiting breaking devices so that the current limiting component and the breaker are in series, in response to excess currents such as a short circuit with sharp build ups, while they have the superior characteristic of being able to limit the current from the first wave, as the width of variation of the inherent resistance is comparatively small, the following areas requiring resolution remain.
(1) As a constant load current flows always through the current limiting component, the current carrying capacity is restricted. Namely, as the temperature of the current limiting component rises proportional to the load current, it is necessary to control the steady temperature of the current limiting component (current carrying current valve) to the extent that it does not have an effect on the circuit and the current limiting characteristics.
(2) As a constant load current flows through the current limiting component, Joule heating (power loss) occurs due to the steady current carrying current.
(3) Under actual conditions, the practicable width of the inherent resistance variation of current limiting components under normal temperatures is approximately 1:10. Therefore, as in a conventional construction where the current limiting component is connected in series between the power source and the load, the higher the voltage of the applicable circuit becomes, the lower the current limiting effectiveness. Namely, the necessary resistance value (Rm) of the current limiting component when current limiting is EQU Rm=E/Im (.OMEGA.) (1)
Here, E is the Circuit Voltage (V), and Im is the current limiting current value (A). A current limiting component resistance value (Ro) when not energizing, is 1/10 of (Rm), Ro=Rm/10. Therefore, the continuous current carrying current allowable value I is EQU I=[.alpha..multidot..theta.t/{Ro(1+.beta..multidot..theta.t)}]1/2 (A)(2)
Here, .alpha. is the heat loss coefficient (W/K) of the current limiting component, .beta. is the resistance temperature coefficient of the current limiting component and .theta.t is the allowable temperature rise value (K) when current limiting component is in a steady state.
As can be understood from the above results, in the case where the current limiting current value (Im) is constant, when circuit voltage (E) increases, it becomes necessary for the resistance value (Rm) of the current limiting component when limiting a current to be correspondingly large, and the steady resistance value (Ro) increases. The heat loss coefficient (.alpha.) of the current limiting component and the steady allowable temperature rise value (.theta.t) is determined by the external shape and the characteristics of the current limiting component. However, if this is constant, the larger the steady resistance value when steady (Ro) of the current limiting component, the lower the continuous current carrying current allowable value (I) becomes. Conversely, if the steady resistance value of the current limiting component is determined with preference to the current carrying current value, as the resistance value of the current limiting component when current limiting will only rise by a factor of 10, the current limiting current value will of necessity rise in conjunction with the rise in circuit voltage and the current limiting effectiveness will decrease.